In a phylogenetic tree, the occurrence of a split in an ancestral branch into more than two branches at an internal node, because the order in which the progenic branches occur cannot be determined.
科学参考收录了60776条科技类词条，基本涵盖了常见科技类参考文献及英语词汇的翻译，是科学学习和研究的有利工具。